The Method Of Reducing Coherent Noise Based On Phase Modulated Interferometer

With the rapid development of precision manufacturing industry, the demanded measurement precision is becoming higher and higher. Because of the advantage of principle, the interferometer has already become an indispensable part in precision measurement. However, the coherent signal is inevitably affected be the coherent noise because the light source used in interferometer is always with strong coherence. Thus, how to suppress the coherent noise in the interference system has become a problem which cannot be ignored.To solve this problem above, in this paper, a method of reducing the coherent noise based on phase modulated interference system is proposed. The coherence of light source is adjusted by phase modulation. Therefore, the coherent signal can be set at the peak of the coherence function and the coherent noise can be set at the valley of the coherence function. As a result, the coherent noise is reduced without damaging the coherent signal. In this paper, several main aspects are accomplished below.(1) Based on the phase modulation theory and wave optics formulas, the principle of forming the partially coherent light by phase modulation is proposed. The mathematical model of the coherent coefficient is built. And the position of the coherent peak and valley are deduced. Based on the principle of the coherent function, the mathematical of noise reduction is built.(2) The relation between the coherence coefficient and the modulation depth, the relation between the optical path difference, and the relation between the synthesized coherence coefficients are simulated respectively. For static and dynamic interferometers, the Twyman interferometer and the grating interferometer are illustrated. The forming reason of the spurious reflection is discussed. The affect of the coherent noise and effect of the noise reduction are simulated. The simulation results show that by using the phase modulation, the coherent noise in Twyman interferometer is reduced by 98.80%; and the coherent noise in grating interferometer is reduced by 81.53% averagely.(3) The modulation driving source with corresponding requirements are designed and manufactured. To begin with, the parameters requirements are analyzed. Then several modules are manufactured such as the RF signal module, the Amplifier module, the Voltage controlled attenuator module, and the square wave shape controlling voltage. The driving source has an excellent performance with frequency 2~4 GHz and power 15.51 d Bm at most.(4) At first, the principle of forming the partially coherent light by phase modulation is verified by Michelson interferometer. Then, the Twyman interferometer and the grating interferometer are built to verify the affect of the coherent noise and effect of the noise reduction. Experimental results show that, by using the phase modulation, the coherent noise pattern is significantly suppressed in the Twyman interferometer. Under the case of equal-path-optical system, the coherent noise is reduced by 50.44%; and under the case of unbalanced path optical system, the coherent noise is reduced by 36.82%.According to the results of simulations and experiments, the method proposed to reduce the coherent noise based on phase modulated interference systems is reasonable and effective.